1. Field of the Invention
The present invention relates to a circuit for heightening a linearity in a power amplifying circuit of a CDMA system, and more particularly, to an RF active balun circuit for improving a small-signal linearity by using an active element balun.
2. Description of the Prior Art
A communication method having a larger capacity than the existing method has been required to meet the explosive demand for greater and more efficient communication. As a result, a digital system capable of accommodating more users was invented.
This digital system is classified into a time division multiple access(TDMA) and a code division multiple access(CDMA). The CDMA system has a merit for the maximum accommodating capacity. A main subject for linearity is generally to heighten the linearity of a power amplifying circuit since the most serious problem for the linearity is caused in the power amplifying circuit used to amplify a signal.
However, the CDMA system uses a diffusion spectrum, in which several users communicate within the same channel frequency. Therefore, the linearity is needed not only in the power amplifying circuits but also in other hardware, more and more in comparison with other methods. In a CDMA receiving terminal the maximum power inputted to an antenna is xe2x88x9225 dBm according to IS-95 as a standard of the CDMA terminal, so the linearity of a small signal in the receiving terminal circuit becomes an important standard.
There are several methods of heightening a linearity of an amplifier in a system level. For example, a predistortion method, xe2x80x9cCuber Predistortion Linearizer for Relay Equipment in 800 MHz Band Land Mobile Telephone Systemxe2x80x9d by T. Nojima et al., in IEEE Trans. Veh., vol. VT-34, No. 4, Nov. 1985, pp. 169-177, is described to generate a third-order distortion signal and appropriately control its phase and size so as to offset it by a third-order distortion signal generated in case that there is no predistortion.
As a first embodiment of such conventional technique, FIG. 1 represents a circuit diagram using a predistortion method for improving linearity.
With reference to FIG. 1, input and output terminals in this circuit each have a divider 1 for providing mutually different transfer paths of a predistortion signal and an existing signal and a combiner 5 for combining two signals. The path of the predistortion signal is composed of a third-order distortion generator 2, a signal attenuator 3 for controlling a magnitude of a signal so that the magnitude of the third-order distortion signals becomes same after the combination of the signals and their phases are inverted to thereby gain a mutual offset between the distortion signals, and a phase shifter 4 for controlling a phase.
As a second embodiment of the conventional technique, FIG. 2 shows a circuit diagram using a feed-forward-cancellation method for improving the linearity, which is xe2x80x9cFeedforward Linearization of 950 MHz Amplifiersxe2x80x9d by R. D. Stewart et al., in IEEE Proceedings, vol.135, No.5, October 1988, pp.347-350.
Referring to FIG. 2, in an input terminal of an auxiliary amplifier, a linear signal of the input terminal is divided through a coupler 6, and the divided signal is transferred to the input terminal of the auxiliary amplifier 9 through a phase shifter 7 and a coupler 8. An output signal amplified in a main amplifier 10 is transferred to the input terminal of the auxiliary amplifier 9 through a coupler 11, a signal attenuator 12 and the coupler 8. At this time, the original signal is cancelled by controlling the signal attenuator 12 and the phase shifter 7 and the auxiliary amplifier 8 amplifies only the third-order distortion signal. The third-order distortion signal outputted through the auxiliary amplifier is combined with the signal of the main amplifier 10 through the coupler 6, and a offset of the third-order distortion signal occurs by controlling an amplification extent of the auxiliary amplifier and a phase of the phase shifter 7 to thus improve the linearity.
As a third embodiment of the conventional technique, FIG. 3 shows a circuit using the cartesian-feedback method to improve the linearity, xe2x80x9cBackoff Improvement of an 800 MHz GaAs FET Amplifier for a QPSK Transmitter using an Adaptive Nonlinear Distortion Cancellerxe2x80x9d by M. Minowa et al., in 40th Veh Tech. Conf., 1990, pp. 542-546.
Referring to FIG. 3, a signal outputted from a main amplifier 15 is feedback through a coupler 16, an auxiliary amplifier 17, a coupler 18 and a phase shifter 19 to an input terminal thereof. Herewith, the third-order distortion signal is cancelled by controlling a phase in the phase shifter 19.
The above conventional circuits have an effect in improving the linearity, but as shown in FIGS. 1 through 3, the circuits may become very complicated, which may also cause the circuits of a big size not suitable for a use of a terminal.
In another conventional technique a transistor-level circuit technique is to offset a phase distortion in a cascade amplifier by using such a characteristic that a phase change between an input signal and its corresponding output signal becomes mutually opposite in field effect transistors(FET) of a common-source terminal and a common-gate terminal, as disclosed in xe2x80x9cA Self Phase Distortion Compensation Technique for Linear Power Amplifiersxe2x80x9d by H. Hayashe et al., in 1994 Asia Pacific Microwave Conf., 1994, pp.555-558. However, the phase change of a scattering coefficient S21 is the phenomenon represented from a large signal, such a signal is applied to a power amplifier and not the cascade amplifier.
Further, Tanaka had utilized such a technique that a cancellation of a third-order distortion signal in an output terminal is made by connecting in parallel two FETs having bias so as to operate in two operation regions by use of a characteristic that a third-order distortion signal component of transconductance most largely influencing upon an FET nonlinearity characteristic becomes different in its sign at a saturation region and a triode region. This technique is disclosed in the article, xe2x80x9cA Linearization Technique for CMOS RF Power Amplifiersxe2x80x9d, by S. Tanaka et al., in Symp. On VLSI Digest, 1997, pp.93-94. But it is not a general characteristic that the third-order distortion signal component of the trans-conductance in the triode region becomes different concerning its signs in the saturation region and the triode region, as disclosed in the article, xe2x80x9cModeling MESFET""s for Intermodulation Analysis of Mixers and Amplifiersxe2x80x9d by S. A. Mass et al., in IEEE Trans. Micro. Tech., vol. 38, December 1990, pp.1964-1971. Therefore this method is only applicable to a specific case.
Such conventional methods are almost limited to a power element for use of a base station, the power element being dull to a difficult degree, size and power consumption of a circuit to be embodied. Even though it is the method for the sake of the small size, low power and high efficiency of the circuit, this effect is provided from the large signal operation. That is, the conventional methods may be not directly applicable to the embodying circuits for improving a linearity of a small signal.
Accordingly, the present invention is directed to an RF active balun circuit for improving a small-signal linearity that substantially obviate one or more of the limitations and disadvantages of the related art.
A primary object of the present invention is to provide an RF active balun circuit for improving a linearity of an IC operating by a small-signal or a medium power by use of a nonlinearity characteristic difference according to gate voltage of an FET, maintaining a terminal characteristic based on a small size, lower power and high efficiency.
To achieve the above object of the present invention, the RF active balun circuit for improving a linearity of a small-signal comprises a signal amplifier driven by exterior individual direct current power, for receiving a communication input signal and performing a cascade amplification at a normal operation point where a feedback third-order distortion signal becomes large; a distortion signal generator driven by exterior direct current power different from the above power, for generating a communication input signal as a third-order distortion signal by a nonlinearity of an active element in order to cancel the third-order distortion signal amplified in the signal amplifier; and an insulator provided for an insulation from the exterior driving power applied to the distortion signal generator.
The signal amplifier is made up of at least two common-source FETs. The at least two common-source FETs are connected in a cascade and used as a main amplification source of a signal. Also, the distortion signal generator uses at least one common-source FET having a drain connected to a drain of said one common-source FET and a gate connected between a source of said one common-source FET and a drain of said another common-source FET. The insulator uses a capacitor.
Under such construction of the present invention, the smaller a third-order differential value g3 of the gate voltage for the drain current in the FET is, the more the linearity of the third-order distortion signal is improved. That is to say, the third-order intermodulation(IP3) is improved by using a characteristic of the FET, wherein the characteristic is that when a consumption of current is small, namely when a linear gain of a signal is small, the magnitude of g3 becomes large, and that when a consumption of current is large, in other words, when the linear gain of the signal is large, the magnitude of the signal becomes small.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof, as well as the appended drawings.